I don't think it's 100%. If I remember it correctly, it must pass into your blood, which means you must have injured mucosa at your genitals. That's why is anal sex much more dangerous because there is higher risk of such damages.

It also depends upon gender. Passage from men to women does not depend on direct blood access (virus in semen can get into lymph drainage once they are far enough into the system), but passage from women to men does require some sort of lesion for access.

Men-to-men does not require lesions, although this was thought to be true - it turns out that the virus can move through some of the rectal surfaces.